This invention relates to oil and rock drilling bits and, more particularly, to the use of composite compacts of diamond, cubic boron nitride (CBN), or wurtzite boron nitride (WBN) in shaped configurations for use as cutting elements for rock drilling.
Drag type rotary bits are commonly fabricated using natural diamond crystals. These bits are used for hard abrasive drilling in deep formations. Such drilling is typically characterized by slow penetration rates (2 to 4 ft./hr.) and long bit life (up to 300 hrs.). Because of the thermal sensitivity of diamonds and the necessity for cooling and cleaning of the individual cutters, good fluid hydraulics are essential to economic bit performance. To obtain acceptable levels of bit hydraulics, diamond bit fabricators have historically used low cutter exposure levels of the individual diamond stones. Conventional diamond drag bits contain individual surface set stones which have exposures or engagements into the rock of the order of 1/16 inch maximum.
It has been recently proposed to use synthetic diamond compacts both cluster and composite as the cutting elements in rotary bits. Such compacts are preferably made in accordance with U.S. Pat. No. 3,745,623.
The advent of such drill blanks, e.g., a sintered diamond layer intimately bonded to a cobalt cemented tungsten carbide layer, has provided a rock cutting tool which permits much more aggressive cutting of hard sandy shales and other abrasive formations. Although drag bits fabricated from these diamond compact blanks are capable of faster penetration rates and equivalent or longer life than diamond drag bits, achievement of optimum performance is often limited by the adherence of the shale, which is under confining pressure, to the sintered diamond cutting surface. Such sticking or loading up of the individual cutters leads to reduced penetration rates and overheating of the cutters, thereby creating reduced abrasion resistance and shortened bit life.
Drag bits fabricated using diamond compacts have historically exhibited much higher exposure levels and thus greater potential engagement of the rock workpiece. These cutters are known to machine the rock which exhibits plastic deformation under the confining pressures which exist at typical well depths of 5,000 to 15,000 or more feet. These cutters are placed on the bit usually at a negative 5.degree. to negative 25.degree. angle of rake of the cutting edge. The cutting edges are usually round or straight and the chips of plastic rock are forced up the flat surface of the sintered diamond layer. In order to provide cleaning and cooling of this sintered diamond surface, necessary to maintain a sharp cutting edge, the bit is designed to channel the drilling mud in a sweeping mode across the cutter surface. In practice it is usually difficult to provide this type of hydraulic action when the desired cutter exposure of 1/4 inch or more is present.